Cladding-pumped fiber devices, such as fiber lasers and amplifiers, are used in a wide range of optical applications in fields such as medicine and surgery, scientific instrumentation, semiconductor device manufacturing, military technology, and industrial material processing. Cladding-pumped fiber lasers and amplifiers can provide high-power and high-quality laser beams and can be implemented in compact, reliable and cost-effective fiber lasers and amplifiers.
Cladding-pumped optical fibers, such as double-clad optical fibers, generally include a core that carries the light signal, a pump or inner cladding surrounding the core and carrying the pump light, and an outer cladding surrounding the pump cladding. The core, pump cladding and outer cladding are made of materials with different refractive indices, such that the index of the core is higher than that of the pump cladding, which, in turn, is higher than the index of the outer cladding. Both core and pump cladding are typically made of silica glass (SiO2). In the gain region thereof, the core is doped with a laser-active dopant material, for example a rare earth such as ytterbium (Yb), erbium (Er) or thulium (Tm). The pump cladding has a large cross-sectional area as compared to the core and high numerical aperture. The outer cladding confines the pump light inside the pump cladding and is commonly made of a low-index polymer rather than glass.
Advantageously, cladding-pumped fibers allow generating a high-brightness and high-quality light signal using low-brightness and low-quality optical pump light. This allows laser diodes to be used as optical pump sources even though they emit high-power laser beams of low brightness. For example, currently-available high-power laser diodes emitting at pump wavelengths in the range from 915 to 980 nanometers (nm) can readily be used for optically pumping rare-earth-doped fiber lasers. The pump light can easily be coupled into the pump cladding due to its large cross-sectional area and high numerical aperture while the light signal propagates in the core. This pumping method is often referred to as “cladding pumping” as opposed to the more conventional “core pumping”, in which the pump light is coupled into the core. Using cladding pumping, fiber lasers can emit laser beams carrying several kilowatts of optical power along with a near diffraction-limited beam quality.
As fiber lasers and amplifiers have been evolving toward higher optical power levels and pulse energies, much attention has been devoted to their reliability, including thermal robustness, optical efficiency and power handling capabilities. A significant parameter affecting their reliability at high optical power levels is the coupling and conversion efficiencies of the pump light in the amplification process. Indeed, when scaling to higher output powers, dissipating or otherwise managing the residual pump power that could not be absorbed in the gain medium becomes important. For example, high-power pump diodes emitting in the 915-980 nm wavelength range can generate significant residual pump power. If not properly stripped or radiated away from the pump cladding, this residual pump power may deteriorate the quality of the light signal as well as the integrity of the optical elements disposed downstream of the active region.
The problem discussed above of stripping residual pump power from the pump cladding of cladding-pumped optical fibers has been considered, for example in U.S. Pat. Nos. 7,373,070 and 7,839,901, in U.S. Patent Application Publication No. 2011/0110625, and in Wetter et al., “High power cladding light strippers”, Proc. of SPIE vol. 6873, p. 687327, (2008).
It is known in the art that pump stripping in a cladding-pumped optical fiber may be achieved first by removing, along a lengthwise segment of the optical fiber, the low-index outer cladding, thereby exposing the inner pump cladding. This lengthwise segment may then be recoated with a transparent coating having a refractive index higher than that of the pump cladding, the high-index coating thus acting as a pump stripper that couples the pump light out of the pump cladding. The extraction length and the magnitude of the dissipated pump power are functions, among other factors, of the difference between the refractive indices of the pump cladding and the high-index coating, and on the way this difference varies with temperature.
U.S. Pat. No. 7,839,901 discloses a lightguide including a light stripper and a fiber having a core and an inner cladding guiding respective light signals. The light stripper includes a coating upon the inner cladding, where the refractive index of the coating is greater than that of the inner cladding. In addition, the refractive index of the coating is selected so that the heat generated upon removal of substantially the entire light from the inner cladding is insufficient to cause a critical temperature rise that would damage the coating. It is mentioned that the refractive index of the coating may be uniform along the entire stripping region of the inner cladding, but that it may alternatively be varied. For example, in one embodiment, the coating may be configured with a succession of sub-regions having respective refractive indices, which differ from one another while remaining higher than the refractive index of the inner cladding.
Wetter et al. in “High power cladding light strippers”, Proc. of SPIE vol. 6873, p. 687327 (2008) also disclose that light stripping in a double-clad fiber can be achieved by recoating the double-clad fiber with a high-index coating. More specifically, they disclose that a homogeneous high-index polymer cannot be used over the entire length of the stripping region in order to minimize localized heating caused by stripping too close to the input end of the stripping region. Rather, a high-index polymer which would gradually strip the light would be ideal to spread the heat load uniformly along the stripper length. Furthermore, it is disclosed that the refractive index of the high-index polymer should increase along the length of the stripper to gradually strip the pump light. They mention that different methods can be used in order to tend toward this situation, such as applying different polymers having different indices at different locations.
In the pump stripping arrangements disclosed in U.S. Pat. No. 7,839,901 and in Wetter et al., varying the value of the refractive index of the high-index outer cladding disposed over a lengthwise segment of the pump cladding upon removal of the original low-index outer cladding therefrom helps ensuring that the pump light is is uniformly stripped along the length of the pump stripper, resulting in a more uniform heat distribution.
The high-index outer cladding of the pump stripping arrangements discussed in the preceding paragraph remains in contact with the low-index outer cladding at the ends of the lengthwise segment. As a result, localized stripping occurring near the junction between the low and high-index outer claddings can cause heating of the low-index outer cladding, which is typically made of a low-index polymer such as acrylate. The role of the low-index outer cladding is to reliably confine the pump light in the pump cladding under the temperature conditions experienced during operation of the device. However, the material entering in the composition of the low-index outer cladding is generally not well adapted to withstand the heat generated therein or therearound upon removal of the residual pump power. For example, the maximum operating temperature of acrylate is around 85 degrees Celsius (° C.), which can be well below the localized temperature reached at the junction between the low and high-index outer claddings as a result of light stripping. Therefore, structural and optical degradation of the low-index outer cladding may occur.
In light of the above, there therefore remains a need in the art for a light stripper operative to effectively dissipate a residual power carried by a pump light propagating in an optical fiber component in a manner that reduces the heat load and associated thermal degradation sustained by the low-index outer cladding of the optical fiber component.